to override searchable-scrollback 's M-s key
one must edit this file:
https://github.com/exg/rxvt-unicode/blob/0d541d78770b5d95e566d56616db060e4308569f/src/perl/searchable-scrollback#L6
that line which seems to be inside a comment
#:META:BINDING:M-s:start

has effect even if the filename is changed
as long as it's not negated in URxvt*perl-ext-common:
like: -searchable-scrollback
(notice the - )

if it's not negated, and it's say renamed to a random file, it will still have effect!!! so M-s or whatever you put there will work! even though it's not explicitly loaded in URxvt*perl-ext-common

solution: actually remove that line
then just override it(set the hotkey) through 
URxvt*keysym.M-d: searchable-scrollback:start

this ^ still gives a warning:
urxvt: perl extension 'searchable_scrollback' not found in perl library search path
but it is loaded and works!!

also remove these lines:
https://github.com/exg/rxvt-unicode/blob/master/src/perl/searchable-scrollback#L39-L44


----
xrdb ~/.Xresources && urxvt
Registering perl extension 'clipboard'
Registering perl extension 'keyboard-select'
Registering perl extension 'option-popup'
Registering perl extension 'readline'
Registering perl extension 'searchable-scrollback'
urxvt: perl extension 'searchable_scrollback' not found in perl library search path
Registering perl extension 'selection'
Registering perl extension 'selection-popup'
Registering perl extension 'url-select'

with this code in urxvt.pm
around line 724 +-
         if (@files) {
            #these still end up in ~/.xsession-errors even with print!
            print "Registering perl extension '$ext'\n";
            $TERM->register_package (extension_package $files[0], $ext_arg{$ext});                                                                              
         } else {
            warn "perl extension '$ext' not found in perl library search path\n";
         }

